The monitoring of eating quality of fruit and vegetables by objective methods is usually destructive and slow (eg squeezing juice to measure Brix). There is a need to be able to rapidly and non-invasively evaluate the internal quality of fresh fruit and vegetables which may enter the fresh market channels or be used for processing. Near infra-red spectroscopy (NIRS), nuclear magnetic resonance (NMR), and acoustic techniques all offer potential for the non-invasive assessment of the internal composition of intact fruit. Of the above NIRS is the most advanced technique with regard to instrumentation, applications, accessories and chemometric software packages, and with respect to cost and speed of operation.
Near infra-red light (NIR) is a small part of the spectrum of electromagnetic radiation—which starts at high-energy waves such as x-rays, through the visible spectrum, to low energy waves such as microwaves and radio waves. NIR is next to the visible portion of light and is a natural part of sunlight. NIR is typically defined as radiation of wavelengths 700–2500 nm, which is invisible to the human eye.
Specific molecules can absorb specific wavelengths. This characteristic is useful in the identification and quantification of a given compound. Absorption of ultraviolet and visible light is associated with the transition of electrons between orbitals in an atom or molecule. Absorption of infra-red radiation by biological material principally involves the rotation and stretching of N—H, C—H and O—H bonds. These bonds are associated with constituents of interest to fruit quality evaluation, including sugar, starch, protein, lipids and water. The infra-red absorption spectrum is commonly used by organic chemists to fingerprint organic molecules. However, ultraviolet, visible and infra-red radiation have poor penetration through bulk tissue. In essence, techniques relying on these wavelengths are useful for solutions or for surface studies. Fortunately, the near infra-red region of the electromagnetic spectrum does penetrate relatively well through bulk biological material, and this region also carries an ‘echo’ of information of the infrared absorption spectrum. Unfortunately, the second and third overtone absorption bands which occur in the near infra-red region are weak and broad, and thus NIRS have been a ‘slow starter’ relative to other techniques, and are only now achieving widespread commercial application.
NIRS technology has been trialed in horticultural applications for over 20 years. However, recent developments in fibre optics, array detectors computing power of PCs and of software capable of carrying out the complex statistical mathematics has made application to inline fruit packing possible.
Past proposals involve the use of either reflectants spectroscopy, with a detector viewing an illuminated area of the fruit, or partial transmittance spectroscopy with the detector mounted at an angle away from the detector. The detector is commonly an array spectrometer, allowing spectral analysis to determine characteristics of the fruit.
Past proposals involve use of tungsten halogen light sources as strong emitters of NIR or NIR emitting diode lasers, at wavelengths chosen relative to the band assignments of the character of interest. For example, U.S. Pat. No. 5,708,271 employs NIR lasers directed at the center of the fruit at an angle of other than 0° and 180°, and commonly 40° to 60°, to the line of the detector and fruit center.